The present invention relates to a scanning lens having a lens element formed with a diffractive lens having a plurality of annular zones on a surface of the lens element, and the diffractive lens formed on the lens element.
Conventionally, a diffractive lens having a lens element formed with a diffractive lens structure thereon has been widely used for various purposes. Examples of the diffractive lenses are disclosed in Japanese Patent Provisional Publications No. HEI 10-197820 (hereinafter, referred to as '820 publication) and No. HEI 11-095145 (hereinafter, referred to as '145 publication). In each of '820 publication and '145 publication, a scanning optical system including a diffractive lens as a scanning lens is disclosed. The scanning optical system is configured such that, a beam emitted by a laser source is deflected by a deflector such as a polygonal mirror or a galvano mirror to scan within a predetermined angular range. The scanning beam is converged, by an fθ lens, on a surface to be scanned (e.g., a surface of a photoconductive drum) and a beam spot is formed thereon. The beam spot moves on the surface of the photoconductive drum in a main scanning direction as the deflector rotates. By ON/OFF modulating the laser beam as it travels in the main scanning direction, a electrostatic latent image is formed on the surface of the photoconductive drum. Since the photoconductive drum is rotates synchronously with the main scanning operation (i.e., an auxiliary scanning operation is performed), a two-dimensional latent image is formed on the surface of the photoconductive drum.
Recently, the laser printers are required to process data rapidly. It is also required that color images are printed by the laser printers. For such purposes, a multi beam scanning optical system or a tandem scanning optical system configured to deflect a plurality of beams emitted by a plurality of light sources using a single deflector (e.g., a polygonal mirror) is used. When a plurality of light sources are used, however, if the wavelengths of the laser beams emitted by the plurality of light sources are different from each other, due to longitudinal chromatic aberration provided by the optical system, a scanning width error occurs. That is, a plurality of scanning lines formed by the scanning beams are different from each other due to the longitudinal chromatic aberration. The optical scanning system disclosed in '820 publication or '145 publication employs a diffractive lens structure that compensates for chromatic aberration on a surface of a scanning lens element.
Generally, the scanning lens used in laser printers are formed by plastic injection molding. However, if the scanning lens element formed with the diffractive lens structure as disclosed in the '820 publication or '145 publication by the plastic injection molding, when the lens itself contracts when demolded from the molding, stepped surfaces between adjacent annular zones and portions therearound may be distorted. As a result, the resultant shape of the diffractive lens structure may be different from the designed shape, that is, different from the molding, and the diffractive efficiency may be significantly lowered. In particular, at a peripheral portion where the width of each annular zone is relatively narrow and density of the annular zones are relatively high, the effect of the distorted shape is significant. When such a lens is used, the intensity of light at a peripheral portion is lowered, which results in deterioration of printed images.